Engine exhaust systems for turbocharged engines commonly include the turbocharger arranged upstream in an exhaust flow direction from the exhaust aftertreatment devices (e.g., catalysts). Such an arrangement, while suitable for fast turbocharger response during some conditions, can lead to increased emissions during cold start conditions due to exhaust heat loss through the turbine of the turbocharger. Further, the exhaust backpressure created by the aftertreatment devices results in increased turbine outlet pressure, reducing the efficiency of the turbocharger.
Other attempts to address the issue of compromised emissions due to heat loss through the turbine include an aftertreatment device closely coupled to the engine. One example approach is shown by Bennet et al. in U.S. Pat. No. 8,276,366. Therein, a plurality of aftertreatment devices are coupled in a housing having multiple flow paths to allow flow of exhaust through one or more of the aftertreatment devices and a turbine of a twin turbocharger. Depending on operating conditions, the exhaust may flow through a turbine prior to flowing through one or more of the aftertreatment devices, or the exhaust may flow through one of the aftertreatment devices prior to flowing through a turbine.
However, the inventors herein have recognized potential issues with such systems. As one example, in every possible flow path in the housing of Bennet, exhaust always flows through at least one aftertreatment device after flowing through a turbine. Thus, the system of Bennet still suffers from the increased turbine outlet pressure that results from subsequent exhaust flow through downstream aftertreatment devices. As a further example, when a flow path is selected that routes exhaust from the engine directly to a turbine and then through one or more aftertreatment devices, it results in one of the aftertreatment devices (an oxidation catalyst) being bypassed altogether. Thus, at least in some examples, emissions may still be comprised. Further still, in Bennet, exhaust always flows through a turbine before flowing through a particulate filter, and thus particulate matter may impinge on the turbine blades, eventually leading to turbine degradation.
In one example, the issues described above may be addressed by a method for an exhaust system of an engine, including during a first condition, flowing a first portion of exhaust gas to a turbine, from the turbine to at least one aftertreatment device, then from the at least one aftertreatment device to atmosphere, and flowing a second portion of exhaust gas to the at least one aftertreatment device, bypassing the turbine, then from the aftertreatment device to atmosphere. The method further includes, during a second condition, flowing a third portion of exhaust gas to the at least one aftertreatment device, from the at least one aftertreatment device to the turbine, and then from the turbine to atmosphere, and flowing a fourth portion of exhaust gas to the at least one aftertreatment device, and then from the at least one aftertreatment device to atmosphere, bypassing the turbine.
In one example, the first condition may include engine output above a first threshold output, for example, during engine peak power and/or load conditions. The second condition may include engine output below a second threshold output, for example, during an idle engine condition. In this way, responsive to peak power and/or load conditions, the exhaust system may be operated to provide desired turbine response during engine peak load conditions by flowing exhaust gas to the turbine, and simultaneously reducing the turbine load to prevent turbine over-speeding by directing at least a part of exhaust gas to the at least one aftertreatment device, bypassing the turbine. Additionally, responsive to the idle engine conditions, the exhaust system may be operated to ensure adequate catalyst warm-up to reduce emissions by flowing exhaust through the at least one aftertreatment device, and simultaneously directing at least a portion of exhaust to flow from the at least one aftertreatment device to atmosphere, bypassing the turbine, reducing pumping losses.
In both the first and second conditions, at least some exhaust still flows through the turbine and all exhaust still flows through the at least one aftertreatment device, and thus no trade-off between emissions and turbine response is required. Further, by maintaining the turbocharger physically between the engine and the at least one aftertreatment device, packaging challenges that result from placing the aftertreatment devices before the turbocharger can be avoided. Further still, if the at least one aftertreatment device includes a particulate filter, by flowing exhaust through the particulate filter before the turbine, at least during some conditions, particulate matter impingement on the turbine may be reduced, increasing the life of the turbine.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.